Within the context of the present disclosure, the expression “internal combustion engine” encompasses Otto-cycle engines, diesel engines and also hybrid internal combustion engines, which utilize a hybrid combustion process, and hybrid drives which comprise not only the internal combustion engine but also an electric machine which may be connected in terms of drive to the internal combustion engine and which receives power from the internal combustion engine or which, as a switchable auxiliary drive, additionally outputs power.
In the development of internal combustion engines, it is a basic aim to minimize fuel consumption, wherein the emphasis in the efforts being made is on obtaining good overall efficiency.
Fuel consumption and thus efficiency pose a problem in particular in the case of Otto-cycle engines, in which the demanded load or power is set by varying the charge of the combustion chamber, that is to say by quantity regulation. However, quantity regulation by a throttle flap has thermodynamic disadvantages in the part-load range owing to the throttling losses.
One approach to a solution for dethrottling the Otto-cycle engine is for example, an Otto-cycle engine operating process with direct injection. The injection of fuel directly into the combustion chamber of the cylinder is considered to be a suitable measure for noticeably reducing fuel consumption even in Otto-cycle engines. The dethrottling of the internal combustion engine is realized by virtue of quality regulation being used within certain limits. Accordingly, by way of direct injection, it is possible to realize a stratified combustion chamber charge. The use of an at least partially variable valve drive likewise offers the possibility of dethrottling. The cylinder deactivation, that is to say the deactivation of individual cylinders in certain load ranges, likewise serves for dethrottling the Otto-cycle engine. The efficiency in part-load operation can be improved, that is to say increased, by way of a partial deactivation because the deactivation of one cylinder of a multi-cylinder internal combustion engine increases, in the case of constant engine power, the load on the other cylinders which remain operational. During the partial deactivation, the cylinders which are permanently in operation furthermore operate in the region of higher loads, at which the specific fuel consumption is lower. The load collective is shifted toward higher loads.
A further measure for improving the efficiency of an internal combustion engine and/or for reducing the fuel consumption consists of supercharging of the internal combustion engine, wherein supercharging is primarily a method of increasing power, in which the air required for the combustion process in the engine is compressed, whereby a greater mass of air can be supplied to each cylinder per working cycle. In this way, the fuel mass and therefore the mean pressure can be increased.
Supercharging is a suitable mechanism for increasing the power of an internal combustion engine while maintaining an unchanged swept volume, or for reducing the swept volume while maintaining the same power. In any case, supercharging leads to an increase in volumetric power output and a more expedient power-to-weight ratio. If the swept volume is reduced, it is thus possible to shift the load collective toward higher loads, at which the specific fuel consumption is lower. By supercharging in combination with a suitable transmission configuration, it is also possible to realize so-called downspeeding, with which it is possible to achieve a lower specific fuel consumption.
Supercharging consequently assists in the constant efforts in the development of internal combustion engines to minimize fuel consumption, which is to say to improve the efficiency of the internal combustion engine.
For supercharging, use is generally made of an exhaust-gas turbocharger, in which a compressor and a turbine are arranged on the same shaft. The hot exhaust-gas flow is supplied to the turbine and expands in said turbine with a release of energy, as a result of which the shaft is set in rotation. The energy supplied by the exhaust-gas flow to the turbine and ultimately to the shaft is used for driving the compressor which is likewise arranged on the shaft. The compressor delivers and compresses the charge air supplied to it, as a result of which supercharging of the cylinders is obtained. A charge-air cooling arrangement may additionally be provided in the intake system downstream of the compressor, by which the compressed charge air is cooled before it enters the cylinders.
The advantage of an exhaust-gas turbocharger in relation to a mechanical charger is that no mechanical connection for transmitting power exists or is required between the exhaust-gas turbocharger and internal combustion engine. While a mechanical supercharger extracts the energy required for driving it entirely from the internal combustion engine, and thereby reduces the output power and consequently adversely affects the efficiency of the engine, the exhaust-gas turbocharger utilizes the exhaust-gas energy of the hot exhaust gases.
The advantage of a mechanical supercharger in relation to an exhaust-gas turbocharger consists in that the mechanical supercharger generates, and makes available, the required charge pressure at all times, specifically regardless of the operating state of the internal combustion engine, in particular regardless of the present rotational speed of the crankshaft. This applies, in particular, to a mechanical supercharger which can be driven by way of an electric machine.
Problems are encountered in the configuration of the exhaust-gas turbocharging, wherein it is basically sought to obtain a noticeable performance increase at all engine speed ranges. In the case of supercharged internal combustion engines with an exhaust-gas turbocharger, a relatively severe torque drop is observed when a certain engine speed is undershot.
Said torque drop is understandable if one takes into consideration that the charge pressure ratio is dependent on the turbine pressure ratio. For example, if the engine speed is reduced, this leads to a smaller exhaust-gas mass flow and therefore to a lower turbine pressure ratio. As a result, the charge pressure ratio likewise decreases in the direction of lower engine speeds, which equates to a torque drop.
Previously, a variety of measures have been used to enhance the torque characteristic of an exhaust gas-turbocharged internal combustion engine, including a small turbine cross section and provision of an exhaust-gas blow-off facility. To this end, a turbine may be equipped with a bypass line which branches off from the exhaust-gas discharge system upstream of the turbine and in which a shut-off element is arranged. Such a turbine is also referred to as a wastegate turbine. If the exhaust-gas mass flow exceeds a threshold value, a part of the exhaust-gas flow is conducted past the turbine, that is to say is blown off, via a bypass line during the course of the so-called exhaust-gas blow-off. This procedure has the disadvantage that the high-energy blown-off exhaust gas remains unutilized and the supercharging behavior is often insufficient at higher engine speeds or in the case of relatively high exhaust-gas quantities.
The torque characteristic of the supercharged internal combustion engine may also be enhanced by multiple turbochargers arranged in parallel, for example, by multiple turbines of relatively small turbine cross section arranged in parallel. The turbines may be activated successively with increasing exhaust-gas flow rate.
The torque characteristic may also be influenced by connecting multiple exhaust-gas turbochargers in series. In one example, connecting two exhaust-gas turbochargers in series, wherein a first exhaust-gas turbocharger serves as a high-pressure stage and a second exhaust-gas turbocharger serves as a low-pressure stage, the compressor characteristic map may be expanded to include both smaller compressor flows and larger compressor flows.
In particular, with the first exhaust-gas turbocharger, which serves as a high-pressure stage, it is possible for the surge limit to be shifted in the direction of smaller compressor flows; because of which high charge pressure ratios may be obtained even with small compressor flows, which may considerably enhance the torque characteristic in the lower engine speed range. This is achieved by using the high-pressure turbine for small exhaust-gas mass flows and by providing a bypass line by which, with increasing exhaust-gas mass flow, an increasing amount of exhaust gas is conducted past the high-pressure turbine. For this purpose, the bypass line branches off from the exhaust-gas discharge system upstream of the high-pressure turbine and opens into the exhaust-gas discharge system again upstream of the low-pressure turbine, wherein a shut-off element is arranged in the bypass line in order to control the exhaust-gas flow conducted past the high-pressure turbine.
The downsizing effect is further enhanced by way of multi-stage supercharging by exhaust-gas turbochargers. Furthermore, the response behavior of an internal combustion engine supercharged in this way is considerably improved in relation to a similar internal combustion engine with single-stage supercharging, because the relatively small high-pressure stage is less inert, and the rotor of a smaller-dimensioned exhaust-gas turbocharger can be accelerated more rapidly.
The European patent EP 1 640 596 B1 discloses an internal combustion engine having two exhaust-gas turbochargers arranged in series, of which a first exhaust-gas turbocharger serves as a low-pressure stage and a second exhaust-gas turbocharger serves as a high-pressure stage. The turbocharger system includes a valve system having valve members that are independently controllable so as to selectively control the gas flow into the turbine portions of the high-pressure turbocharger and the low-pressure turbocharger units.
However, the inventors herein have recognized that the turbocharger system described in EP 1 1640 596 does not provide active control of high boost pressure to the engine over a wide range of speed and load conditions. Further measures may be desired in order to improve the torque characteristic and in order to increase efficiency in order to satisfy the future demands placed on a modern exhaust-gas-turbocharged internal combustion engine. In particular, supercharging concepts are of interest with which the surge limit can be shifted further toward even lower charge-air flow rates in order to improve the torque characteristic of the internal combustion engine at very low engine speeds. Furthermore, an ever faster response of the supercharging arrangement is demanded in order to improve the transient behavior of the internal combustion engine. The latter is in particular also of relevance in conjunction with exhaust-gas recirculation. Furthermore, it is basically always the case that a high maximum power or large power increase is sought.
The inventors herein have recognized the above cited potential issues, and provide systems and methods to at least partly address the issues. In one example, a supercharged internal combustion engine system comprises an intake system for supply of charge air to an internal combustion engine; an exhaust-gas discharge system for discharge of exhaust gases from the internal combustion engine; at least two series-connected exhaust-gas turbochargers which each comprise a turbine arranged in the exhaust-gas discharge system and a compressor arranged in the intake system, the at least two series-connected exhaust-gas turbochargers including a first exhaust-gas turbocharger that serves as a low-pressure stage and a second exhaust-gas turbocharger that serves as a high-pressure stage, a second compressor of the second exhaust-gas turbocharger arranged downstream of a first compressor of the first exhaust-gas turbocharger, a first turbine of the first exhaust-gas turbocharger arranged downstream of a second turbine of the second exhaust-gas turbocharger; a third bypass line in which a third shut-off element is arranged; a third turbine arranged in the exhaust-gas discharge system in parallel with respect to the second turbine of the second exhaust-gas turbocharger, the third turbine equipped with a variable turbine geometry and connected in terms of drive to a generator; a fourth shut-off element for activation purposes arranged upstream of the third turbine, a first bypass line in which a first shut-off element is arranged and which branches off from the exhaust-gas discharge system upstream of the third turbine and the second turbine of the second exhaust-gas turbocharger and which opens into the exhaust-gas discharge system again downstream of the first turbine and the second turbine; a third compressor arranged in the intake system between the first compressor of the first exhaust-gas turbocharger and the second compressor of the second exhaust-gas turbocharger and which is connected in terms of drive to an electric motor; and a second bypass line in which a second shut-off element is arranged and which branches off from the intake system between the first compressor of the first exhaust-gas turbocharger and the third compressor and which opens into the intake system between the third compressor and the second compressor of the second exhaust-gas turbocharger.
The turbine of the second exhaust-gas turbocharger, which will hereinafter also be referred to as the second turbine, is in the present case equipped with a bypass line. According to the disclosure, it is additionally the case that a further turbine is provided in the high-pressure stage, which further turbine is arranged in parallel with respect to said second turbine, that is to say the high-pressure turbine. Both turbines of the high-pressure stage can be bypassed via a bypass line, specifically the first bypass line. This makes it possible for the high-pressure turbine to be configured for very low exhaust-gas flow rates.
The associated compressor of the HP exhaust-gas turbocharger is of correspondingly small dimensions, whereby the surge limit is shifted toward very low charge-air flow rates and high charge pressures can be realized in a first operating mode even at very low engine speeds. The torque characteristic of the internal combustion engine in the low engine speed range is improved considerably as a result.
In one example, in the presence of very low charge-air flow rates (e.g. at low engine speed), the boosting system operates as the two-stage conventional turbocharger system in the first operating mode, wherein the additional compressor, first HP turbine, and the second turbine bypass line are deactivated by virtue of the respective valves being closed. That way, the airflow will be compressed by the HP compressor and flow through the second HP turbine in the exhaust system. The very small high-pressure stage of the internal combustion engine is less inert. The relatively fast response of the small high-pressure stage in the event of a load alteration considerably improves the transient behavior of the internal combustion engine.
It should be understood that summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the details description. Furthermore the claimed subject matter is not limited to implementations that solve any disadvantages notes above or in any part of this disclosure.